[show abstract][hide abstract] ABSTRACT: Although the use of ultrasonic plane-wave transmissions rather than line-per-line focused beam transmissions has been long studied in research, clinical application of this technology was only recently made possible through developments in graphical processing unit (GPU)-based platforms. Far beyond a technological breakthrough, the use of plane or diverging wave transmissions enables attainment of ultrafast frame rates (typically faster than 1000 frames per second) over a large field of view. This concept has also inspired the emergence of completely novel imaging modes which are valuable for ultrasound-based screening, diagnosis, and therapeutic monitoring. In this review article, we present the basic principles and implementation of ultrafast imaging. In particular, present and future applications of ultrafast imaging in biomedical ultrasound are illustrated and discussed.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 01/2014; 61(1):102-119. · 1.80 Impact Factor
[show abstract][hide abstract] ABSTRACT: Elasticity maps of tissue have proved to be particularly useful in providing complementary contrast to ultrasonic imaging, e.g., for cancer diagnosis at the millimeter scale. Optical coherence tomography (OCT) offers an endogenous contrast based on singly backscattered optical waves. Adding complementary contrast to OCT images by recording elasticity maps could also be valuable in improving OCT-based diagnosis at the microscopic scale. Static elastography has been successfully coupled with full-field OCT (FF-OCT) in order to realize both micrometer-scale sectioning and elasticity maps. Nevertheless, static elastography presents a number of drawbacks, mainly when stiffness quantification is required. Here, we describe the combination of two methods: transient elastography, based on speed measurements of shear waves induced by ultrasonic radiation forces, and FF-OCT, an en face OCT approach using an incoherent light source. The use of an ultrafast ultrasonic scanner and an ultrafast camera working at 10,000 to 30,000 images/s made it possible to follow shear wave propagation with both modalities. As expected, FF-OCT is found to be much more sensitive than ultrafast ultrasound to tiny shear vibrations (a few nanometers and micrometers, respectively). Stiffness assessed in gel phantoms and an ex vivo rat brain by FF-OCT is found to be in good agreement with ultrasound shear wave elastography.
Journal of Biomedical Optics 12/2013; 18(12):121514. · 2.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: Shear wave imaging (SWI) maps soft tissue elasticity by measuring shear
wave propagation with ultrafast ultrasound acquisitions (10 000 frames
s-1). This spatiotemporal data can be used as an input
for an inverse problem that determines a shear modulus map. Common
inversion methods are local: the shear modulus at each point is
calculated based on the values of its neighbour (e.g. time-of-flight,
wave equation inversion). However, these approaches are sensitive to the
information loss such as noise or the lack of the backscattered signal.
In this paper, we evaluate the benefits of a global approach for
elasticity inversion using a least-squares formulation, which is derived
from full waveform inversion in geophysics known as the adjoint method.
We simulate an acoustic waveform in a medium with a soft and a hard
lesion. For this initial application, full elastic propagation and
viscosity are ignored. We demonstrate that the reconstruction of the
shear modulus map is robust with a non-uniform background or in the
presence of noise with regularization. Compared to regular local
inversions, the global approach leads to an increase of contrast
(˜+3 dB) and a decrease of the quantification error (˜+2%). We
demonstrate that the inversion is reliable in the case when there is no
signal measured within the inclusions like hypoechoic lesions which
could have an impact on medical diagnosis.
[show abstract][hide abstract] ABSTRACT: Supersonic shear wave elastography (SWE) is a quantitative stiffness imaging technique based on the combination of a radiation force induced in tissue by an ultrasonic beam and ultrafast ultrasound imaging sequence (up to more than 10,000 frames per second) catching in real time the propagation of the resulting shear waves. Local shear wave speed is estimated and enables the two dimensional mapping of shear elasticity. This imaging modality is implemented on conventional probes driven by dedicated ultrafast echographic devices and can be performed during a standard ultrasound exam. The clinical potential of SSI is today extensively investigated for many potential applications such as breast cancer diagnosis, liver fibrosis staging, cardiovascular applications, and ophthalmology. This invited lecture will present an overview of the current investigated applications of SSI and the new trends of shear wave elastography research topics.
The Journal of the Acoustical Society of America 11/2013; 134(5):4009. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: We show how disorder can be used to guide a broadband ultrasonic wave. The idea is to exploit the transverse localization regime that has been reported for light. Our waveguide consists of a set of parallel cylindrical scatterers randomly distributed in the transverse plane. An ultrasonic beam propagating along the direction of scatterers is found to remain confined in the two other directions on a size smaller than the waveguide diameter and driven by the localization length. Interestingly, the guided wave is also found to propagate with a very limited temporal dispersion.
[show abstract][hide abstract] ABSTRACT: Shear wave imaging (SWI) maps soft tissue elasticity by measuring shear wave propagation with ultrafast ultrasound acquisitions (10 000 frames s(-1)). This spatiotemporal data can be used as an input for an inverse problem that determines a shear modulus map. Common inversion methods are local: the shear modulus at each point is calculated based on the values of its neighbour (e.g. time-of-flight, wave equation inversion). However, these approaches are sensitive to the information loss such as noise or the lack of the backscattered signal. In this paper, we evaluate the benefits of a global approach for elasticity inversion using a least-squares formulation, which is derived from full waveform inversion in geophysics known as the adjoint method. We simulate an acoustic waveform in a medium with a soft and a hard lesion. For this initial application, full elastic propagation and viscosity are ignored. We demonstrate that the reconstruction of the shear modulus map is robust with a non-uniform background or in the presence of noise with regularization. Compared to regular local inversions, the global approach leads to an increase of contrast (∼+3 dB) and a decrease of the quantification error (∼+2%). We demonstrate that the inversion is reliable in the case when there is no signal measured within the inclusions like hypoechoic lesions which could have an impact on medical diagnosis.
Physics in Medicine and Biology 09/2013; 58(19):6765-6778. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abstract Purpose: Transcranial high intensity focused ultrasound (HIFU) therapy guided by magnetic resonance imaging (MRI) is a promising approach for the treatment of brain tumours. Our objective is to validate a dedicated therapy monitoring system for rodents for transcranial HIFU therapy under MRI guidance in an in vivo brain tumour model. Materials and methods: A dedicated MR-compatible ultrasound therapy system and positioning frame was developed. Three MR-compatible prefocused ultrasonic monoelement transducers were designed, operating at 1.5 MHz and 2.5 MHz with different geometries. A full protocol of transcranial HIFU brain therapy under MRI guidance was applied in n = 19 rats without and n = 6 rats with transplanted tumours (RG2). Different heating strategies were tested. After treatment, histological study of the brain was performed in order to confirm thermal lesions. Results: Relying on a larger aperture and a higher frequency, the 2.5 MHz transducer was found to give better results than other ones. This single element transducer optimised the ratio of the temperature elevation at the focus to the one at the skull surface. Using optimised transducer and heating strategies enabled thermal necrosis both in normal and tumour tissues as verified by histology while limiting overheating in the tissues in contact with the skull. Conclusions: In this study, a system for transcranial HIFU therapy guided by MRI was developed and tested in an in vivo rat brain tumour model. The feasibility of this therapy set-up to induce thermal lesions within brain tumours was demonstrated.
International Journal of Hyperthermia 08/2013; · 2.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: Acoustic focusing experiments usually require large arrays of transducers. It has been shown by Etaix et al. [J. Acoust. Soc. Am. 131, 395-399 (2012)] that the use of a cavity allows reducing this number of transducers. This paper presents experiments with Duralumin plates (the cavities) containing scatterers to improve the contrast of focusing. The use of a scatterer array in the plate allows increasing the modal density at given frequencies. The scatterers used are membranes and buttons that are manufactured in Duralumin plates. Their resonances are studied both experimentally and numerically. Such scatterers present the advantage of having a tunable frequency resonance, which allows controlling the frequencies at which the modal density increases. The dispersion relations of plates with scatterer array show high modal density at given frequencies. Finally acoustic focusing experiments in air, using these plates, are compared to the ones of simple duralumin plates demonstrating the improvement of contrast. Acoustic source localization is also realized using these plates.
The Journal of the Acoustical Society of America 08/2013; 134(2):1049-54. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: High-intensity focused ultrasound causes selective tissue necrosis efficiently and safely, namely, in the prostate, liver, and uterine fibroid. Nevertheless, ablation of brain tissue using focused ultrasound remains limited due to strong aberrations induced by the skull. To achieve ultrasonic transcranial brain ablation, such aberrations have to be compensated. In this study, non-invasive therapy was performed on monkeys using adaptive correction of the therapeutic beam and 3D simulations of transcranial wave propagation based on 3D computed tomographic (CT) scan information. The aim of the study was two-fold: induce lesions in a non-human primate brain non-invasively and investigate the potential side effects. Stereotactic targeting was performed on five Macaca fascicularis individuals. Each hemisphere was treated separately with a 15-day interval and animals were sacrificed two days after the last treatment. The ultrasonic dose delivered at the focus was increased from one treatment location to the other to estimate the thermal dose for tissue alteration. Thermal doses in the brain were determined by numerical computations. Treatment efficiency and safety were evaluated histologically. The threshold for tissue damage in the brain was measured to be between 90 and 280 cumulative equivalent minutes at 43 °C. Intravenous injection of corticoids before the treatment limited the side effects.
The Journal of the Acoustical Society of America 08/2013; 134(2):1632-9. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Purpose: Low-intensity focused ultrasound has been shown to stimulate the brain noninvasively and without noticeable tissue damage. Such a noninvasive and localized neurostimulation is expected to have a major impact in neuroscience in the coming years. This emerging field will require many animal experiments to fully understand the link between ultrasound and stimulation. The primary goal of this paper is to investigate transcranial ultrasonic neurostimulation at low frequency (320 kHz) on anesthetized rats for different acoustic pressures and estimate the in situ pressure field distribution and the corresponding motor threshold, if any. The corresponding acoustic pressure distribution inside the brain, which cannot be measured in vivo, is investigated based on numerical simulations of the ultrasound propagation inside the head cavity, reproducing at best the experiments conducted in the first part, both in terms of transducer and head geometry and in terms of acoustic parameters.Methods: In this study, 37 ultrasonic neurostimulation sessions were achieved in rats (N = 8) using a 320 kHz transducer. The corresponding beam profile in the entire head was simulated in order to investigate the in situ pressure and intensity level as well as the spatial pressure distribution, thanks to a rat microcomputed tomography scan (CT)-based 3D finite differences time domain solver.Results: Ultrasound pulse evoked a motor response in more than 60% of the experimental sessions. In those sessions, the stimulation was always present, repeatable with a pressure threshold under which no motor response occurred. This average acoustic pressure threshold was found to be 0.68 ± 0.1 MPa (corresponding mechanical index, MI = 1.2 and spatial peak, pulse averaged intensity, Isppa = 7.5 W cm(-2)), as calibrated in free water. A slight variation was observed between deep anesthesia stage (0.77 ± 0.04 MPa) and light anesthesia stage (0.61 ± 0.03 MPa), assessed from the pedal reflex. Several kinds of motor responses were observed: movements of the tail, the hind legs, the forelimbs, the eye, and even a single whisker were induced separately. Numerical simulations of an equivalent experiment with identical acoustic parameters showed that the acoustic field was spread over the whole rat brain with the presence of several secondary pressure peaks. Due to reverberations, a 1.8-fold increase of the spatial peak, temporal peak acoustic pressure (Psptp) (±0.4 standard deviation), a 3.6-fold increase (±1.8) for the spatial peak, temporal peak acoustic intensity (Isptp), and 2.3 for the spatial peak, pulse averaged acoustic intensity (Isppa), were found compared to simulations of the beam in free water. Applying such corrections due to reverberations on the experimental results would yield a higher estimation for the average acoustic pressure threshold for motor neurostimulation at 320 KHz at 1.2 ± 0.3 MPa (MI = 2.2 ± 0.5 and Isppa = 17.5 ± 7.5 W cm(-2)).Conclusions: Transcranial ultrasonic stimulation is pressure- and anesthesia-dependent in the rat model. Numerical simulations have shown that the acoustic pattern can be complex inside the rat head and that special care must be taken for small animal studies relating acoustic parameters to neurostimulation effects, especially at a low frequency.
Medical Physics 08/2013; 40(8):082902. · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Optical wavefront-shaping has emerged as a powerful tool to manipulate light
in strongly scattering media. It enables diffraction-limited focusing and
imaging at depths where conventional microscopy techniques fail. However, while
most wavefront-shaping works to-date exploited direct access to the target or
implanted probes, the challenge is to apply it non-invasively inside complex
samples. Ultrasonic-tagging techniques have been recently demonstrated but
these require a sequential point-by- point acquisition, a major drawback for
imaging applications. Here, we introduce a novel approach to non-invasively
measure the optical transmission-matrix inside a scattering medium, exploiting
the photo-acoustic effect. Our approach allows for the first time to
simultaneously discriminate, localize, and selectively focus light on multiple
targets inside a scattering sample, as well as to recover and exploit the
scattering medium properties. Combining the powerful approach of the
transmission-matrix with the advantages of photoacoustic imaging opens the path
towards deep-tissue imaging and light-delivery utilizing endogenous optical
[show abstract][hide abstract] ABSTRACT: We present an experimental demonstration of electromagnetic
Green’s function retrieval from thermal radiations in anechoic and
reverberant cavities. The Green’s function between two antennas is
estimated by cross correlating milliseconds of decimeter noise. We show
that the temperature dependence of the cross-correlation amplitude is
well predicted by the blackbody theory in the Rayleigh-Jeans limit. The
effect of a nonuniform temperature distribution on the cross-correlation
time symmetry is also explored. Finally, we open a new way to image
scatterers using ambient thermal radiations.
[show abstract][hide abstract] ABSTRACT: L’échographie est une modalité d’imagerie médicale qui s’est développée en pratique clinique dans les années 1970. Après l’avènement du mode d’imagerie Doppler, dans les années 1990, les principes d’élastographie sont apparus pour mieux caractériser les propriétés mécaniques des tissus et essayer d’apporter une information quantitative à la palpation qualitative du médecin. Dans cet article sont résumées les principales techniques d’élastographie : du régime quasi-statique nécessitant une simple compression du milieu, au régime dynamique utilisant la propagation des ondes mécaniques dans le corps. Les principes techniques des principales méthodes dynamiques sont explicités : vibro-acoustographie, impulsion de force de radiation acoustique (ARFI), élastographie impulsionnelle, acquisition ultrarapide… Cette synthèse permet au lecteur d’avoir un aperçu sur ces nouvelles méthodes ultrasonores vouées à devenir un outil d’utilisation quotidienne pour le radiologue.
Journal de Radiologie Diagnostique et Interventionnelle. 05/2013; 94(5):504–513.
[show abstract][hide abstract] ABSTRACT: Ultrasonography has been widely used for diagnosis since it was first introduced in clinical practice in the 1970's. Since then, new ultrasound modalities have been developed, such as Doppler imaging, which provides new information for diagnosis. Elastography was developed in the 1990's to map tissue stiffness, and reproduces/replaces the palpation performed by clinicians. In this paper, we introduce the principles of elastography and give a technical summary for the main elastography techniques: from quasi-static methods that require a static compression of the tissue to dynamic methods that uses the propagation of mechanical waves in the body. Several dynamic methods are discussed: vibro-acoustography, Acoustic Radiation Force Impulsion (ARFI), transient elastography, shear wave imaging, etc. This paper aims to help the reader at understanding the differences between the different methods of this promising imaging modality that may become a significant tool in medical imaging.
[show abstract][hide abstract] ABSTRACT: The combination of ultrasound and optics, together with statistics, now
permits light focusing and imaging deep inside strongly scattering media
at the optical diffraction limit.
[show abstract][hide abstract] ABSTRACT: Hemodynamic changes in the brain are often used as surrogates of neuronal activity to infer the loci of brain activity. A major limitation of conventional Doppler ultrasound for the imaging of these changes is that it is not sensitive enough to detect the blood flow in small vessels where the major part of the hemodynamic response occurs. Here, we present a μDoppler ultrasound method able to detect and map the cerebral blood volume (CBV) over the entire brain with an important increase in sensitivity. This method is based on imaging the brain at an ultrafast frame rate (1 kHz) using compounded plane wave emissions. A theoretical model demonstrates that the gain in sensitivity of the μDoppler method is due to the combination of 1) the high signal-to-noise ratio of the gray scale images, resulting from the synthetic compounding of backscattered echoes; and 2) the extensive signal averaging enabled by the high temporal sampling of ultrafast frame rates. This μDoppler imaging is performed in vivo on trepanned rats without the use of contrast agents. The resulting images reveal detailed maps of the rat brain vascularization with an acquisition time as short as 320 ms per slice. This new method is the basis for a real-time functional ultrasound (fUS) imaging of the brain.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 03/2013; 60(3):492-506. · 1.80 Impact Factor
[show abstract][hide abstract] ABSTRACT: Object This work aimed at evaluating the accuracy of MR-guided high-intensity focused ultrasound (MRgHIFU) brain therapy in human cadaver heads. Methods Eighteen heads of fresh human cadavers were removed with a dedicated protocol preventing intracerebral air penetration. The MR images allowed determination of the ultrasonic target: a part of the thalamic nucleus ventralis intermedius implicated in essential tremor. Osseous aberrations were corrected with simulation-based time reversal by using CT data from the heads. The ultrasonic session was performed with a 512-element phased-array transducer system operating at 1 MHz under stereotactic conditions with thermometric real-time MR monitoring performed using a 1.5-T imager. Results Dissection, imaging, targeting, and planning have validated the feasibility of this human cadaver model. The average temperature elevation measured by proton resonance frequency shift was 7.9°C ± 3°C. Based on MRI data, the accuracy of MRgHIFU is 0.4 ± 1 mm along the right/left axis, 0.7 ± 1.2 mm along the dorsal/ventral axis, and 0.5 ± 2.4 mm in the rostral/caudal axis. Conclusions Despite its limits (temperature, vascularization), the human cadaver model is effective for studying the accuracy of MRgHIFU brain therapy. With the 1-MHz system investigated here, there is millimetric accuracy.
Journal of Neurosurgery 03/2013; · 3.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: The ability to control wave propagation is of fundamental interest in
many areas of physics. Photonic crystals proved very useful for this
purpose but, because they are based on Bragg interferences, these
artificial media require structures with large dimensions.
Metamaterials, on the other hand, can exhibit very deep subwavelength
spatial scales. In general they are studied for their bulk effective
properties that lead to effects such as negative refraction. Here we go
beyond this effective medium paradigm and we use a microscopic approach
to study metamaterials based on resonant unit cells. We show that we can
tailor unit cells locally to shape the flow of waves at deep
subwavelength scales. We validate our approach in experiments with both
electromagnetic and acoustic waves in the metre range demonstrating
cavities, waveguides, corners and splitters with centimetre-scale
dimensions, an order of magnitude smaller than previous proposals.
[show abstract][hide abstract] ABSTRACT: The method of the time reversal operator decomposition is usually employed to detect and characterize static targets using the invariants of the time reversal operator. This paper presents a theoretical and experimental investigation into the impact of small displacements of the target on these invariants. To find these invariants, the time reversal operator is built from the multistatic response matrix and then diagonalized. Two methods of recording the multistatic response matrix while the target is moving are studied: Acquisition either element by element or column by column. It is demonstrated that the target displacement generates new significant eigenvalues. Using a perturbation theory, the analytical expressions of the eigenvalues of the time-reversal operator for both acquisition methods are derived. We show that the distribution of the new eigenvalues strongly depends on these two methods. It is also found that for the column by column acquisition, the second eigenvector is simply linked to the scatterer displacements. At last, the implications on the Maximum Likelihood and Multiple Signal Classification detection are also discussed. The theoretical results are in good agreement with numerical and 3.4 MHz ultrasonic experiments.
The Journal of the Acoustical Society of America 01/2013; 133(1):94-107. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this article, we investigate composite media which present both a local resonance and a periodic structure. We numerically and experimentally consider the case of a very academic and simplified system that is a quasi-one dimensional split ring resonator medium. We modify its periodicity to shift the position of the Bragg bandgap relative to the local resonance one. We observe that for a well-chosen lattice constant, the local resonance frequency matches the Bragg frequency thus opening a single bandgap which is at the same time very wide and strongly attenuating. We explain this interesting phenomenon by the dispersive nature of the unit cell of the medium, using an analogy with the concept of white light cavities. Our results provide new ways to design wide and efficient bandgap materials.